Operating instructions

PTQ-MCM ♦ Quantum / Unity Platform Reference
Modbus Communication Module
Page 124 of 139 ProSoft Technology, Inc.
April 29, 2008
In this example, the client addresses a request to server device. The function
code (01) is for a Read Output Status operation. It requests the status of the
output at address 1245 (04A1 hex). Note that only that one output is to be read,
as specified by the number of outputs field (0001).
If the output address is non-existent in the server device, the server will return
the exception response with the exception code shown (02). This specifies an
illegal data address for the slave.
MODBUS Exception Codes
Code Name Meaning
01 Illegal Function
The function code received in the query is not an
allowable action for the server (or slave). This may be
because the function code is only applicable to newer
devices, and was not implemented in the unit selected. It
could also indicate that the server (or slave) is in the
wrong state to process a request of this type, for example
because it is unconfigured and is being asked to return
register values.
02 Illegal Data Address
The data address received in the query is not an allowable
address for the server (or slave). More specifically, the
combination of reference number and transfer length is
invalid. For a controller with 100 registers, a request with
offset 96 and length 4 would succeed, a request with
offset 96 and length 5 will generate exception 02.
03 Illegal Data Value
A value contained in the query data field is not an
allowable value for server (or slave). This indicates a fault
in the structure of the remainder of a complex request,
such as that the implied length is incorrect. It specifically
does not mean that a data item submitted for storage in a
register has a value outside the expectation of the
application program, because the Modbus protocol is
unaware of the significance of any particular value of any
particular register.
04 Slave Device Failure
An unrecoverable error occurred while the server (or
slave) was attempting to perform the requested action.
05 Acknowledge
Specialized use in conjunction with programming
commands. The server (or slave) has accepted the
request and is processing it, but a long duration of time
will be required to do so. This response is returned to
prevent a timeout error from occurring in the client (or
master). The client (or master) can next issue a poll
program complete message to determine if processing is
completed.
06 Slave Device Busy
Specialized use in conjunction with programming
commands. The server (or slave) is engaged in
processing a long-duration program command. The client
(or master) should retransmit the message later when the
server (or slave) is free.